1. Field of the Invention
The present invention relates to a block copolymer composition and sheet.
2. Description of the Related Art
Block copolymers consisting of vinyl aromatic hydrocarbons and conjugated dienes are widely used for various types of packaging such as food product containers, beverage containers, industrial containers (including various types of trays) and blister packs. Packaging often needs to be highly designable, and the content must be clearly visible, so the raw material of the packaging must have transparency and gloss. On the other hand, block copolymers consisting of vinyl aromatic hydrocarbons and conjugated dienes are often mixed with vinyl aromatic hydrocarbon polymers for use in packaging, so such resin compositions also require good transparency. Concerning this problem, Patent Documents 1 and 2 describe that the use of a block copolymer consisting of a vinyl aromatic hydrocarbon having a specific polymer structure and a conjugated diene is effective for solving the problem.
Furthermore, while the strength (tensile strength, impact strength) to withstand actual use is necessary for packaging applications, there are means of using block copolymers with a higher conjugated diene content for the purpose of conferring such physical properties. However, if block copolymers with a high conjugated diene content are used carelessly, the compatibility with the vinyl aromatic hydrocarbon polymers can be reduced, making it difficult to ensure good transparency and gloss.
On the other hand, as a general method of producing packaging, there is a method of melt-kneading the material, forming sheets, and thermoforming such as by vacuum forming, pressure forming or press forming to obtain various types of formed articles. These production processes include off-line systems in which sheets are formed and wound into rolls for storage, then the sheets are unwound from the rolls and thermoformed to obtain the formed articles, and in-line systems in which the sheets are formed and immediately thermoformed without being stored in rolls to obtain the formed articles. In off-line systems, the sheets are stored for a long time in mutual contact in roll form, so when using a block copolymer consisting of vinyl aromatic hydrocarbons and conjugated dienes, the sheets may stick together and become inseparable (hereinafter referred to as “blocking”). If blocking defects occur, the sheets cannot be used in subsequent thermoforming or the like, so anti-blocking properties are sought. Regarding this problem, Patent Documents 3 to 5 describe the use of a hydrocarbon wax in the block copolymer as a means for effectively preventing blocking.
However, depending on the structure of the block copolymer consisting of a vinyl aromatic hydrocarbon and a conjugated diene, there are cases in which sufficient anti-blocking effects cannot be obtained when arbitrarily choosing hydrocarbon waxes for use, and the use of hydrocarbon waxes suitable for the polymers used is desired.
Additionally, if the process involves storing the formed articles by stacking them on top of each other after forming the packaging, the contacting surfaces of the formed articles can stick together (hereinafter referred to as “sticking”) when they are to be separated for use. This problem tends to occur when using a block copolymer consisting of a vinyl aromatic hydrocarbon and a conjugated diene. In particular, when the recesses in the packages are deep or when the shapes of the packages are more complex, the contact area between the formed articles increases and separation becomes more difficult, so good anti-sticking properties are needed. In recent years, the use of packaging with complicated shapes has been increasing, so better anti-sticking properties have been sought in materials.